Field of the Invention
This invention relates to a method of forming a regular solution secondary eutectic nickel- or cobalt-base superalloy -- which contains, on a weight basis, 2-9% rhenium, less than about 0.8% titanium, and optionally an element selected from the group consisting of chromium, aluminum, tantalum, carbon, nickel, cobalt, tungsten, vanadium, molybdenum, niobium, hafnium, zirconium and boron -- which comprises combining at a temperature above the eutectic melt temperature of the superalloy (1) a primary eutectic nickel-base superalloy scrap containing, on a weight basis, 2-9% rhenium, and less than about 0.8% titanium, and optionally said elements; (2) a first master alloy containing the elements nickel, aluminum, vanadium, and carbon; (3) a second master alloy containing the elements nickel and carbon, and (4) elements selected from the group consisting of rhenium, titanium, chromium, aluminum, tantalum, nickel, cobalt, tungsten, molybdenum, niobium, hafnium, zirconium and boron including combinations thereof in alloy form.
Nickel and cobalt base eutectic alloys which can be unidirectionally solidified in the production of metallic composites containing aligned fibers as a reinforcing phase of the superalloy matrix are advantageously employed in gas turbine engines because of their improved high temperature performance characteristics. The cost of such superalloy compositions which contain expensive pure alloy elements, such as rhenium requires that substantially all superalloy casting scrap generated during the formation of the primary superalloy ingot from pure alloying elements, as well as scrap, e.g. gates, risers, etc., generated during the production of the directionally solidified end product, must be remelted to form secondary alloys and be reused. The melt procedure suited to the preparation of the primary eutectic superalloy employed in my invention are not suited to remelt procedures utilizing primary alloy scrap for a portion of the raw materials of a secondary eutectic superalloy because of objectionable molten metal-crucible reactions, i.e. reactions in the liquidus solidus chemistry of eutectic melts during homogenization of eutectic alloys, caused by the presence of such reactive elements as carbon, aluminum, vanadium, etc., in the scrap material which are not normally a part of the initial furnace charge during a virgin melt. These undesirable reactions are related to melt problems such as (1) dissolved gases, e.g. oxygen, etc., (2) dissolved nongaseous elements, e.g. rhenium and tungsten, (3) suspended oxides, e.g. alumina, etc., and (4) inclusions including indigenous, e.g. tantalum carbide, etc., as well as exogeneous impurities.